GB2343848A

GB2343848A - Control unit for prosthesis

Info

Publication number: GB2343848A
Application number: GB9925179A
Authority: GB
Inventors: Richard Vavasour Hammond
Original assignee: AONIC Ltd
Current assignee: AONIC Ltd
Priority date: 1998-11-03
Filing date: 1999-10-26
Publication date: 2000-05-24
Anticipated expiration: 2019-10-26
Also published as: GB2343848B; GB9925179D0; GB9823946D0

Abstract

The unit includes an annular piston 34 which slides within a cylinder 20 and a piston rod 13 which extends from the cylinder. A reservoir space 73 is disposed co-axially about the cylinder bounded by a flexible wall 70 fixed at both ends 71 and 72 such that the mid region of the wall can flex diametrically to define a variable reservoir volume which receives hydraulic fluid displaced from the cylinder 20 by movement of the piston 34. A compression spring 63 contained wholly within the piston rod 13 acts between the piston rod and a fixed feed tube 56 such as to urge the piston rod out of the cylinder. The feed tube contains two groups of axially spaced ports 53 and 54 on opposite sides of the piston, and respective passages conduct hydraulic fluid from said groups of ports to the opposite end of the cylinder via an adjustable restriction and a non-return valve.

Description

PROSTHESIS CONTROL UNIT TECHNICAL FIELD OF THE INVENTION This invention relates to an hydraulic control unit for a prosthetic limb, primarily a leg.

BACKGROUND In the development of prosthetic limbs the ultimate aim is to produce a prosthesis having a control system which faithfully reproduces the characteristics of the muscles and tendons in a real limb. In prosthetic legs, an hydraulic control unit is pivotally connected between the thigh and shin sections to control movement about the knee joint, and more particularly, to provide controlled and adjustable damping during flexion and extension of the prosthesis. The basic form of the device has remained unchanged for many years and most function as described in US Patent No. 2 859 451. A cylinder is mounted within an outer housing which forms a reservoir for hydraulic fluid. A piston carried by a piston rod is arranged to move axially within the cylinder and hydraulic damping of the piston is provided by a series of axially spaced ports at each end of the cylinder which conduct hydraulic fluid through a series of passages to enter the opposite end of the cylinder via an adjustable restriction and a non-return valve.

In addition to providing a damping effect, known control units are arranged to store kinetic energy during flexion and then release the stored energy during extension to swing the limb forward into the correct position for commencing the next stride. In one such unit this is achieved by providing unequal volumes of fluid on opposite sides of the damper piston. The reservoir housing contains a coiled compression spring which acts against an accumulator piston to define a variable reservoir volume. As the damped piston moves into the cylinder during flexion of the prosthesis the difference in fluid volumes is displaced into the fluid reservoir causing the accumulator piston to move to compress the spring, which then acts by fluid pressure to urge the piston rod outwardly and assist the return of the prosthesis to the extended position. With such an arrangement the accumulator piston must be provided with an effective seal to prevent leakage of hydraulic fluid past the piston. Therefore, the return force of the spring must overcome the friction of the seal so that a proportion of the spring return power is wasted.

Such units also suffer from the fact that different seals have variable friction properties so that similar units can exhibit vastly different operating characteristics. The seal friction also results in an undesirable rise in temperature of the unit during extended periods of operation, typically of the order of 40 C.

In addition to providing the functions described above, the control units are also required to be of compact size, lightweight, and give a long and reliable service life with a minimum amount of maintenance.

US Patent No. 5 092 902 discloses a form of hydraulic control unit in which the spring and accumulator piston are replace with a gas-charged bladder mounted within the piston rod. As the piston rod moves into the cylinder the bladder is pressurised to provide an increasing gas pressure which acts on the reservoir of fluid to extend the piston rod during the return stroke.

Although such an arrangement would have reduced friction there is always a problem in retaining a fixed charge of gas over a prolonged period. In addition, gas pressure changes considerably with temperature so that it would again be difficult to provide consistent operating characteristics under different operating conditions.

The present invention seeks to provide a new and inventive form of hydraulic control unit for use with a prosthesis.

SUMMARY OF THE INVENTION The present invention proposes a control unit for a prosthesis, comprising: -a cylinder ; -a piston assembly comprising a piston movable within the cylinder and a piston rod extending from the cylinder ; -a reservoir for receiving hydraulic fluid displaced from the cylinder by movement of the piston assembly into the cylinder; -a flexible wall bounding at least part of said reservoir to define a variable reservoir volume ; and -a compression spring acting on the piston assembly such as to urge the piston rod out of the cylinder.

The return force is provided primarily by the compression spring which operates with no appreciable friction and exhibits consistent characteristics.

The flexible wall acts principal to accommodate changes in fluid volume, with the elimination of sliding seals. The outside surface of the flexible wall, which is on the opposite side to the reservoir fluid, need not be subjected to a high pressure, although the wall could act to compress a contained volume of gas at relatively low pressure so that the gas pressure would, to a minor extent, assist the return spring. In fact, it is perfectly possible for the outside surface of the flexible wall to be open to atmospheric pressure, but in any event, the wall is not required to retain a high gas pressure.

The flexible wall could, for example, be in the form of a dish-shaped diaphragm, but in order to provide a more compact arrangement the flexible wall is preferably substantially cylindrical and fixed at both ends such that the mid region of the wall can flex diametrically. The reservoir is preferably of a substantially cylindrical configuration disposed co-axially about the cylinder so that the length of the unit is minimised.

The compression spring may be disposed on the opposite side of the piston to the piston rod, but again, a more compact unit can be achieved if the spring is housed at least partly within the piston rod. A further reduction in the length of the unit may be achieved if the spring is contained wholly within the piston rod and acts against a fixed member which extends through the piston.

It will be appreciated that, in common with most known control units, the control unit of the invention can include provision for damping movements of the piston. Said provision for damping movements of the piston preferably comprises two groups of axially spaced ports on opposite sides of the piston and respective passages which conduct hydraulic fluid from said groups of ports to the opposite end of the cylinder via an adjustable restriction and a non-return valve. In a preferred arrangement the control ports are contained within said fixed member, thereby providing more space around the piston in which to accommodate the reservoir.

BRIEF DESCRIPTION OF THE DRAWINGS The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings: Figure 1 is a general view of a prosthetic leg incorporating an hydraulic swing phase control unit in accordance with the invention; Figure 2 is an exploded general view of the control unit; Figure 3 is a longitudinal section through the housing and main cylinder of the control unit; Figure 4 is a further longitudinal section through the control unit in the region of the adjustable flow restrictors; Figure 5 is a longitudinal section through a first non-return valve of the control unit; Figure 6 is a section detail at the upper end of the housing; and Figure 7 is a longitudinal section through a second non-return valve of the control unit.

DETAILED DESCRIPTION OF THE DRAWINGS Referring to Fig. 1, the prosthetic leg 1 comprises a polymeric thigh portion 2 which is formed as a socket for receiving the stump of a amputee's leg.

The lower end of the thigh portion 2 has a metal fitting 3 which incorporates a rearwardly projecting bracket 4. A shin portion 5, typically of carbon fibre, is pivotally connected to the fitting 3, in front of the bracket 4, by a hinge pin 6 to form a knee joint 7, and an artificial foot 8 is pivotally connected to the lower end of the shin portion 5 in known manner. The upper portion 9 of the shin portion is typically formed as a hollow U-section which accommodates an hydraulic swing phase control unit 10, shown separated from the prosthesis for convenience of illustration. The control unit inclues a machined housing 11 formed with a trunnion 12 at its lower end for pivotal connection with the shin portion 5, and a piston rod 13 projects from the upper end of the housing, provided with a second trunnion 14 for pivotal connection with the bracket 4.

Referring now to Fig. 2, which shows the control unit 10 in more detail, the housing 11 of the control unit 10 comprises a main part 111 of generally rectangular section having a bottom end cap 112 which incorporates the trunnion 12. The upper end of the housing is also provided with a top cap 31. The top and bottom caps 31 and 112 are held to the main part 111 by four bolts 113 (only one of which is shown) which pass axially through the four corners of the housing. The housing part 111 contains a cylinder 20 (indicated in outline only), first and second non-return valves 21 and 22 respectively, and first and second adjustable flow restrictors 23 and 24 respectively.

As shown in Fig. 3, the cylinder 20 is located within a chamber 30 in the housing part 111, retained by the end cap 31. A washer 32, interposed between the lower end of the cylinder and the end cap 112, provides the necessary fluid seals at the lower end of the cylinder 20. An annular piston 34 is axially slidable within the cylinder 20, dividing the space within the cylinder into upper and lower fluid chambers, 35 and 36 respectively. The radially outer surface of the piston 34 is provided with a sliding seal 38 between wear rings 39 and 40. The piston is connected with the tubular piston rod 13, the upper fluid chamber 35 surrounding the piston rod being in communication with the interior space 41 of the piston rod via ports 42 adjacent to the piston 34. The upper end of the piston rod 13 is closed by a plug 43 which is formed with the trunnion 14. A guide bush 44 is located within the upper end of the cylinder 20 to slidably guide the piston rod 13, the bush being located by an annular flange 45 held between the upper end of the cylinder 20 and the end cap 31. The upper chamber 35 of the cylinder 20 is sealed by an O-ring 46 interposed between the end of the cylinder and the end cap 31, and by a lip seal 47 located within the cap 31 in sealing contact with the piston rod 13. A wiper seal 48 within the cap 31 protects the assembly from entry of dirt carried on the piston rod.

An orifice tube 50, sealed to the end cap 112 at the washer 32, projects through the piston 34 in sealing contact with a piston bush 51. The upper end of the orifice tube 50 is closed by an end wall 52. The wall of the orifice tube contains two series of axially spaced control ports 53 and 54 positioned at the lower and upper ends of the tube respectively. Within the orifice tube is a feed tube 56. There is an annular space 57 between the feed tube 56 and the orifice tube 50, the upper end of the feed tube being sealed to the inside of the orifice tube at 58 between the ports 53 and 54. The lower end of the feed tube 56 is sealed to the end cap 112 by an O-ring 59 such that the annular space 57 communicates with drillings 60 and the interior space 61 of the feed tube communicates with further drillings 61.

The piston rod 13 contains a compression spring 63, shown in outline only, which bears against the end wall 52 and the plug 43, thus acting to extend the piston rod 13 from the housing 11.

Continuing to refer to Fig. 3, it can be seen that the outer surface of the cylinder 20 is formed with an annular recess 66 having a cylindrical base 67 located between part-conical regions 68 and 69. The recess 66 receives a flexible wall in the form of a generally cylindrical elastomeric sleeve 70. The two ends of the sleeve are formed with sealing beads 71 and 72 which are sealably held between the cylinder 20 and the housing part 111, whereas the mid region of the sleeve is free to flex radially within the recess 66. The space between the sleeve 70 and the cylinder 20 thus forms fluid accumulator 73 of variable volume. The opposed space 74 between the sleeve 70 and the housing part 111 may be open to atmosphere but is preferably sealed at a low gas pressure to prevent entry of dirt and provide a small positive pressure to assist the return of fluid accumulated in the space 73.

Moving on now to Fig. 4, the first and second adjustable flow restrictors 23 and 24 are of substantially identical construction. Essentially, each restrictor comprises a sleeve 78 which is rotatable by an external knob 79. The sleeve is internally threaded for engagement with a threaded portion 80 at one end of an adjuster needle 81. The adjuster needle is prevented from rotating by a tubular screw 85 located in the housing part 111 and having a hexagonal bore 86 which slidably receives a hexagonal stem portion 87 between the adjuster needle 81 and the threaded portion 80. Thus, rotation of the respective knob 79 causes fine axial movement of the needle 81 to vary flow of hydraulic fluid through a control bobbin 82 between an input port 83 and an outlet port 84. The bobbin 82 of the first adjuster 23 receives fluid from the drillings 60 whereas the second adjuster 24 receives fluid from the drillings 61, the output of both adjusters being in fluid communication with the accumulator space 73.

The first non-return valve 21 is shown in Fig. 5. Fluid output from the first adjuster 23 can pass through the valve by moving a ball 91 away from a valve seat 92 formed in a bobbin 93. As shown in Fig. 6, the output from the valve 21 is returned to the space surrounding the piston rod 13 via drillings 96 in the housing 11, drilling 97 and groove 98 in the bush 44.

The second non-return valve 22, shown in Fig. 7 receives fluid flow from the second adjuster 24 causing a valve piston 101 to move away from a seat 102 formed in a respective bobbin 103. Fluid output from the valve 22 is returned to the fluid space at the inner end of the piston 13 via a drilling (not shown) in the washer 32 (Fig. 3).

It will be appreciated that any convenient form of check valve could be used for the valves 21 and 22.

From the foregoing it should now be apparent that the control unit operates as follows. Retraction of the piston rod 13 caused by flexure of the prosthetic leg about the knee joint forces hydraulic fluid through the ports 53, annular space 57 and via drillings 60 to the adjuster 23, which thus controls the damping resistance of the unit during flexure. The fluid is returned to the opposite side of the piston via the drillings 96,97 and groove 98, excess fluid being accumulated in the space 73 by outward movement of the flexible sleeve 70. At the end of the retraction stroke the return spring 63 acts to urge the piston out of the cylinder 20, the return movement of the piston causing fluid to flow through the ports 54, feed tube 56 and drillings 61, via extension damping resistance adjuster 24 and non-return valve 22 to return to the cylinder via washer 32. Excess hydraulic fluid is of course also returned from the accumulator space 73 via the non-return valve 22.

It will be appreciated that during both strokes the non-return valves 21 and 22 act to prevent a reverse flow of fluid to that described. Furthermore, the reducing number of outlet ports 53 or 54 which are operative towards the ends of the piston stroke provide increased damping resistance, in known manner.

It will thus be appreciated that the use of a return spring and a fluid accumulator without dynamic seals provides a control unit having very consistent and reliable performance. Tests have shown that the temperature rise of the unit over a prolonged working period is much less than with known units (14 C against 40 C). It will be appreciated that the features disclosed herein may be present in any feasible combination. Whilst the above description lays emphasis on those areas which, in combination, are believed to be new, protection is claimed for any inventive combination of the features disclosed herein.

Claims

CLAIMS 1. A control unit for a prosthesis, comprising: -a cylinder ; -a piston assembly comprising a piston movable within the cylinder and a piston rod extending from the cylinder ; -a reservoir for receiving hydraulic fluid displaced from the cylinder by movement of the piston assembly into the cylinder ; -a flexible wall bounding at least part of said reservoir to define a variable reservoir volume; and -a compression spring acting on the piston assembly such as to urge the piston rod out of the cylinder.
2. A control unit according to Claim 1, in which the flexible wall is substantially cylindrical and fixed at both ends such that the mid region of the wall can flex diametrically.
3. A control unit according to Claim 2, in which the reservoir is of a substantially cylindrical configuration disposed co-axially about the cylinder.
4. A control unit according to any preceding claim, in which the compression spring is housed at least partly within the piston rod.
5. A control unit according to Claim 4, in which the compression spring is contained wholly within the piston rod and acts against a fixed member which extends through the piston.
6. A control unit according to any preceding claim, which includes provision for damping movements of the piston.
7. A control unit according to Claim 6, in which said provision for damping movements of the piston comprises two groups of axially spaced ports on opposite sides of the piston and respective passages which conduct hydraulic fluid from said groups of ports to the opposite end of the cylinder via an adjustable restriction and a non-return valve.
8. A control unit according to Claim 7 as appended to Claim 5, in which the control ports are contained within said fixed member.
9. A control unit for a prosthesis, substantially as described with reference to the accompanying drawings.